Electrochemical cells provide electrical energy that power a host of electronic devices ranging from medical devices to electronic devices utilized in gas and oil exploration. Among these many devices powered by electrochemical cells are electronic devices such as pipeline inspection gauges that are used in down-hole petroleum exploration. Such devices generally require the delivery of a significant amount of current over long periods of time in relatively harsh environments. Thus, these devices typically require the use of electrochemical cells, such as lithium oxyhalide electrochemical cells, that provide increased delivery capacity and rate of charge delivery. In addition, these cells must be able to safely operate in harsh environments that may comprise elevated temperatures, increased atmospheric pressures, caustic environments, explosive atmospheres, or combinations thereof.
Since these cells power devices that typically operate in remote, harsh environments, it is ideal to know ahead of time when a cell is reaching its end-of-life and, thus, requires replacement. A depleted cell could result in loss of data and/or lost operating time due to a non-operating device. Therefore, it is ideal to have a reliable end-of-life indicator to optimize the timing of cell removal and minimize down time.
As defined herein, “delivery capacity” is the maximum amount of electrical current that can be drawn from a cell under a specific set of conditions. The terms, “rate of charge delivery” and “rate capability” are defined herein as the maximum continuous or pulsed output current a battery can provide per unit of time. Thus, an increased rate of charge delivery occurs when a cell discharges an increased amount of current per unit of time in comparison to a similarly built cell, but of a different anode and/or cathode chemistry.
A traditional lithium oxyhalide electrochemical cell comprises an anode composed of lithium and a cathode composed of an electrochemically non-active carbon material, such as acetylene black. In addition, a traditional lithium oxyhalide cell comprises a catholyte that is typically composed of an electrochemically active cathode material and liquid electrolyte. In general, the catholyte is composed of a highly oxidizing liquid, i.e., thionyl chloride or sulfuryl chloride that also serves as the electrochemically active cathode material. Because the catholyte provides both the electrochemically active cathode material and the electrolyte in a lithium oxyhalide cell, the cell volume is generally efficiently utilized. As a result, lithium oxyhalide cells typically have energy densities on the order of about 700 to about 1200 Wh/L in comparison to primary cells of other chemistries. The generally high energy densities and the ability of such batteries to operate under extreme temperature conditions make lithium oxyhalide electrochemical cells ideally suited to power a wide range of devices used in extreme environments that require a long discharge life.
As defined herein, “non-active” means that the material does not undergo an electrochemical reaction within an electrochemical cell. For example, acetylene black is non-active within an oxyhalide cell as it does not chemically react with the liquid oxyhalide catholyte. The term “active” means that the material undergoes an electrochemical reaction within an electrochemical cell. The term “catholyte” means an ionically conductive solution that is operatively associated with the anode and the cathode.
During discharge of a traditional lithium oxyhalide electrochemical cell, the active cathode material within the catholyte undergoes an electrochemical reduction reaction with lithium supplied by the anode. This reduction reaction typically continues at a constant rate until either of the catholyte or the anode lithium material is exhausted. As a result, the voltage of a lithium oxyhalide cell generally remains stable throughout discharge until end-of-life is reached. Because the discharge voltage typically remains stable throughout cell discharge, monitoring the discharge voltage is not an ideal indicator that the cell is nearing end-of-life.
In other primary electrochemical cell chemistries, the operating voltage of the cell typically decreases as the cell is discharged. Therefore, the operating voltage can be used as an indicator of a cell's state of discharge. The cell discharge voltage in these other chemistries can therefore be used to provide a warning that the cell is nearing the end of its useful life, thus allowing the cell to be replaced before failure occurs. However, because the operating voltage in lithium oxyhalide cells generally remains substantially constant, monitoring discharge voltage does not provide a reliable indicator about the state of discharge. Thus, there is a risk that the cell may abruptly reach end-of-life with little warning, thereby possibly resulting in unexpected device failure and/or data loss.
Therefore, other embodiments of monitoring the state of discharge of lithium oxyhalide cells have been developed. One such embodiment of monitoring the state of discharge of a lithium oxyhalide cell is measuring the amount of energy that has been extracted from the cell during discharge. Various electronic circuits have been developed to monitor energy that is consumed during cell discharge. One of the most common of these electronic techniques is Coulomb counting. In Coulomb counting, the state of discharge is determined by measuring the amount of current flow. Specifically, a circuit is connected to the cell that measures the voltage drop across a resistor in the circuit. However, electric circuitry such as a Coulomb counting circuit consumes energy from the cell and, thus, becomes a parasitic load that reduces available energy. Another problem with electronic circuits is that the electronic components that comprise these circuits typically become unreliable at increased temperatures. This is particularly problematic at the elevated temperature conditions in which lithium oxyhalide cells typically operate.
A second approach is to design the lithium oxyhalide cell with an additional secondary cathode. These cells comprise a second cathode having a higher impedance than the primary cathode. Therefore, when these cells near end-of-life, the secondary cathode delivers a lower voltage as compared to the discharge voltage previously delivered. The drop in discharge voltage that typically occurs at the end-of-life of the cell is thus used as an indicator of approaching end-of-life. An example of such a cell construction is disclosed in U.S. Pat. No. 5,569,553 to Smesko et al., which is assigned to the assignee of the present invention and incorporated herein by reference. Additional examples of electrochemical cells comprising secondary cathodes can also be found in U.S. Pat. No. 4,293,622 to Marincic et al. and U.S. Pat. No. 4,563,401 to Kane et al., both of which are herein incorporated by reference. In practice, however, such cell designs have proven to be ineffective because measurable voltage differences that are useful for indicating an end-of-life condition are only observed under limited discharge conditions that typically do not occur in most cases of lithium oxyhalide cell use.
A third approach is to include a secondary anode material within the anode construction. The secondary anode material is designed to discharge at a lower voltage after the primary lithium metal anode material has been consumed. U.S. Pat. No. 4,416,957 to Goebel et al., incorporated herein by reference, describes the use of a calcium metal as a secondary anode material. This cell design provides a lower discharge voltage step near end-of-life after the primary anode lithium metal has been consumed. In practice, however, it has been found that anodes comprising calcium metal discharge poorly in an oxyhalide catholyte and, thus, do not establish a reduced voltage that is stable for a long enough period of time to act as a reliable end-of-life indicator.
Yet another approach is to include a secondary catholyte material in the cell that discharges against the remaining lithium metal anode material after the first liquid cathode material, e.g., thionyl chloride, has been consumed. U.S. Pat. No. 4,371,592 to Gabano, incorporated herein by reference, describes the addition of phosphoryl chloride and benzoyl chloride, respectively, as secondary active catholytes. These cells are designed to discharge at a lower voltage after the thionyl chloride active electrolyte has been consumed. In practice, however, these secondary liquid cathode materials are known to form complex intermediate species through reaction with the first liquid cathode materials, such as thionyl chloride and associated discharge products. Thus, as a result, the drop in discharge voltage is typically not significant enough to discern between discharge states and act as a reliable indicator of cell end-of-life.
In general, the inclusion of a secondary anode or cathode material reduces the discharge voltage of a lithium oxyhalide cell. In addition, the additional anode or cathode material occupies volume within the cell that could otherwise be occupied by materials that exhibit increased discharge voltage. Thus, as a result, the total energy and energy density of the cell is reduced than would otherwise be available in the absence of these lower voltage materials.
Therefore, what is needed is a lithium oxyhalide cell that provides increased energy density with a distinct indication of approaching end-of-life. More specifically, a lithium oxyhalide cell that provides increased energy density and a distinct indication of approaching end-of-life that does not rely on external electronic circuitry or comprise a secondary anode, cathode or electrolyte having a reduced discharge voltage within a lithium oxyhalide cell. Thus, the present invention addresses these problems by providing a cathode material composition and cathode thereof for a lithium oxyhalide electrochemical cell having an increased discharge capacity, energy density and further provides a discernible end-of-life indicator.